1. Field of the Disclosure
The invention relates to an apparatus and method for separating solids from a solids laden drilling fluid and, more particularly, but not exclusively, a vibratory separator commonly known as a shale shaker for separating solids from a solids laden drilling mud.
2. Description of the Related Art
In the drilling of a borehole in the construction of an oil or gas well, a drill bit is arranged on the end of a drill string, which is rotated to bore the borehole through a formation. A drilling fluid known as “drilling mud” is pumped through the drill string to the drill bit to lubricate the drill bit. The drilling mud is also used to carry drill cuttings produced by the drill bit and other solids to the surface through an annulus formed between the drill string and the borehole. The density of the drilling mud is closely controlled to inhibit the borehole from collapse and to ensure that drilling is carried out optimally. The density of the drilling mud affects the rate of penetration of the drill bit. By adjusting the density of the drilling mud, the rate of penetration changes at the possible detriment of collapsing the borehole. The drilling mud may also carry lost circulation materials for sealing porous sections of the borehole. The acidity of the drilling mud may also be adjusted according to the type of formation strata being drilled through. The drilling mud contains inter alia expensive synthetic oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires inter alia certain solids to be removed from the drilling mud. This is achieved by processing the drilling mud. The first part of the process is to separate drill cuttings from the solids laden drilling mud. This is at least partly achieved with a vibratory separator, such as those shale shakers disclosed in U.S. Pat. No. 5,265,730, WO 96/33792 and WO 98/16328. Further processing equipment such as centrifuges and hydrocyclones may be used in the processing of the used drilling mud. The solids are covered in contaminates and residues. It is not uncommon to have 30 to 100 m3 of drilling fluid in circulation in a borehole.
The resultant solids, known herein as “drill cuttings” are processed to remove substantially all of the residues and contaminates from the solids. The solids can then be disposed of in a landfill site or by dumping at sea in the environment from which the solids came. Alternatively, the solids may be used as a material in the construction industry or have other industrial uses.
Shale shakers generally comprise an open bottomed basket having one open discharge end and a solid walled feed end. A number of rectangular screens are arranged over the open bottom of the basket. The screens may be substantially planar or have a slight crown. The basket is arranged on springs above a receptor for receiving recovered drilling mud. A skip or ditch is provided beneath the open discharge end of the basket. A motor is fixed to the basket, which has a drive rotor provided with an offset clump weight. In use, the motor rotates the rotor and the offset clump weight, which causes the basket and the screens fixed thereto to shake. Solids laden mud is introduced at the feed end of the basket on to the screens. The shaking motion induces separation of the drilling mud from the solids, the drilling mud passing through the screens and the solids above the screens. The shaking motion also induces the solids to move along the screens towards the open discharge end. The recovered drilling mud is received in the receptor for return to the active mud system of the drilling rig or on for further processing and the solids pass over the discharge end of the basket into the ditch or skip.
The recovered drilling mud may contain smaller particles such as additives that are required for maintaining desired drilling fluid density and viscosity, which may be desirable and thus these smaller particles are left in the recovered drilling mud.
The rectangular screens may be arranged at an angle to horizontal, such as a seven degrees incline from the feed end to the discharge end of the shale shaker. The angle may be adjustable. The screens are generally fixed in the basket and the basket is adjustable to adjust the angle of the screens relative to horizontal. The flow of solids laden drilling fluid may form a pool on the inclined screens. The action of the vibratory mechanism induces solids to climb the inclined screens to the discharge end of the shaker and into the ditch or skip.
Generally, a vibratory mechanism inducing a circular vibration will tend to throw solids from the screen into the air in generally circular motions sufficient to move the overflow of solids along a generally horizontal screen deck.
A vibratory mechanism inducing an elliptical motion will induce the solids to move in a direction of the longest chord of the ellipse, which is often used in shale shakers having an inclined screen deck, such that the solids move up the inclined screen deck. A shale shaker having a vibratory mechanism inducing a very thin ellipse is known as a linear shale shaker and induces fast movement of solids along the screen, although the screen tends to suffer fast degradation due to sudden deceleration of the solids as they meet the screen. The screen decks are inclined so that a pool of drilling mud to be screened is allowed to form thereon, which creates a head of fluid above the screen. This head helps push drilling mud and small solids through the screen. The elliptical or linear motion helps solids move along the screen within the pool, on to and traverses a dry portion of the screen, known as a beach. The solids move along the beach over a discharge end of the screen into a cuttings ditch, trough, pipe or other conveying means.
The pool of fluid on the screen deck adds to the weight need to be vibrated. Furthermore, too large a head of drilling mud above the screen may increase the likelihood of blinding the screen, which may well occur when screening drilling mud with heavy clays therein. In particular, the heavy clays may break down on the screen and be pushed into openings in the screen mesh. Thus the pool size and depth can affect efficient operation of the separator or shaker. A pool that is too deep may not be adequately filtered. A pool that is too shallow may flow across a screen too quickly or without sufficient weight to be filtered adequately. It is generally beneficial to have a pool of drilling mud on the screen, but one having a controlled depth.
One factor in screen life is how well the beach area is maintained. “Beach” area is the distance from a fluid-dry interface-with-fluid line on a final screen to the end of the screen. Thus, a zero beach length describes a shaker operating with drilling fluid covering the entire screen area of the final screen and running over to discharge. This can be costly due to the loss of drilling fluid which flows off, rather than through, a screen. A beach length of 50 cm (20 inches) in certain shakers indicates a shaker operating fairly dry, with the last sections of screen potentially vibrating against completely dry particles. Such dry particles vibrating on the beach of the last screen can tear holes in the screen and shorten the screen life. The beach length is affected by variables such as fluid flow rates and drilling fluid properties including viscosity, density, temperature, and solids content.
The screens used in a shale shaker experience high acceleration values, heavy loading, high throughput and variable beach lengths when processing drilling fluids. The high acceleration values are required in order to induce throughput through the screens and in order to convey the solids over the screen for discharge.
The shale shaker vibratory drive apparatus is sized in order to induce an optimal stroke profile based upon the total mass of assembly including the vibratory drive apparatus, basket, screens and solids laden drilling fluid therein i.e. all of the vibrated components, essentially the sprung mass. The screens are clamped or otherwise fastened to the basket and the screens support the solids laden drilling fluid to be processed.
WO 2005/105327, incorporated herein for all purposes, discloses a vibratory separator for screening solids laden drilling mud, the vibratory separator comprising at least one screen arranged in a basket isolated from a base, vibratory apparatus for vibrating the at least one screen and means for adjusting the angle of the at least one screen, such that, in use solids laden drilling mud forms a pool on the at least one screen, the pool having a surface, the surface having a trailing edge defining a beach characterized in that the vibratory apparatus comprises a variable frequency drive (or inverter) and control apparatus to control the variable frequency drive for driving the screen in at least one motion profile. Preferably, the control apparatus comprises a PLC or a computer. If a desired acceleration value is changed according to the states of the shaker and mud, then performance is further enhanced and/or optimized. For instance, the acceleration level is adjusted to minimize energy consumption, or to use the minimum necessary acceleration based on the fluid level in the shaker. This is useful since high acceleration can lead to decreased screen life and solids degradation.
WO 2005/105327 also discloses an automatic boost feature. In this case, the shaker acceleration is temporarily increased to a predetermined value, e.g. in order to accommodate unusually high mud flow. More generally, when the shaker acceleration is controlled, then it can be varied over a continuous range [e.g. for such control a monitoring device monitors an accelerometer on or connected to a shaker basket and the monitoring device (e.g. a computer, PLC, or PID controller) sends a signal to a VFD). For instance, the acceleration can be controlled to be constant under varying mud flow/loading. This is accomplished by measuring the basket acceleration with an accelerometer located on the basket, in one aspect near the center of mass, e.g. accelerometer AC in FIG. 11C; feeding back the signal from the accelerometer to a shaker computer (or PID controller, or to a remote computer); comparing the actual acceleration with the desired value; and changing the motor frequency or other driving force until the actual acceleration matches the desired acceleration.
WO 2007/045924 discloses a shale shaker comprising an electromagnetic vibratory apparatus having a driving apparatus comprising a variable frequency drive to maintain the basket at a natural resonant frequency.
WO 2007/057712 discloses a shale shaker comprising an electromagnetic vibratory apparatus, which preferably is arranged between a floating basket and a fixed base and advantageously comprises a leaf spring.
The motion induced into the assembly is typically a simple harmonic motion, whereby starting from its lowest point the vibratory drive, basket, screens and solids laden drilling fluids is accelerated upwards typically in an elliptical motion, the longest chord of the ellipse set at around forty-five degrees towards the front of the basket by high positive acceleration forces reducing towards mid stroke where the acceleration values reduce to zero then negative deceleration occurs reducing the velocity to a minimum at the top of the stroke. This cycle is repeated on the downward stroke showing that high acceleration values, (positive and negative) are experienced at each tip of the stroke where the velocity is minimized and the acceleration values are zero at each mid-point of the stroke where the velocities are at their highest.
As the fluids are not physically attached to the screens the screen loading varies therefore dependent upon the stroke cycle. During the upward stroke the screen is accelerated into the fluid mass thereby inducing throughput then the screen accelerates away from the fluid mass on the downward stroke only for the fluid mass to drop onto the screens again at the bottom of the stroke due to gravity and the whole cycle is then repeated typically at approximately 1800 to 2000 rpm.
For optimal performance the screen overall should preferably move as one entity matching the stroke profile of the basket and any deflection should be minimized. Should the screen deflect then this can lead to far higher acceleration values being induced leading to premature wear and uneven loading across the width of the screen. Given that the motion is also utilized to convey solids across the screen, any additional deflection of the screen will result in uneven conveyance across the screen width.
It is known to have a further screen deck in the shale shaker, having screens thereon of around 40 to 60 mesh to scalp off large solids. The underflow is then directed back on to a lower, feed end of a primary screen deck. This feature can be found on the VSM 300® Shale Shaker supplied by the Brandt division of National Oilwell Varco. It is also known to have a drying deck, such that the solids overflow from the discharge end of the primary deck on to drying deck to facilitate drying of the solids. This feature can be found on the VSM 300® Shale Shaker and King Cobra® Shale Shaker supplied by the Brandt division of National Oilwell Varco.
It is advantageous to use fine-meshed filters to filter very small particles, for example of a size in the range of 50-200μ or more, without the filtering device clogging up with the small particles. However, it is the fine-meshed filters in particular that are prone to such undesired clogging.
It is also advantageous to provide a separator which operates at low noise levels to comply with health and safety legislation. It is also advantageous to have a simple reliable separator to inhibit down-time for maintenance and repair.
It is preferable in certain circumstances to retain particles, for example of a particle size in the range of 50-60μ or larger, by means of a filter.
Apparatus has been modified to separate out Lost Circulation Material for re-use. Such apparatus comprises a scalping deck to screen out large drill cuttings, the underflow of which flows on to a primary deck provided with mesh sized and designed to screen out Lost Circulation Material. The Lost Circulation Material is returned to the active mud system of the drilling rig and the underflow from the primary screen flows through a secondary screen deck to filter out drill cuttings. The filtered drill cuttings are walked into a trough or cuttings ditch. The underflow of drilling fluid containing very small particles is returned to the active mud system. Such a system is the SWECO Multi-Deck Mud Cleaner, tub type shale shaker supplied by SWECO Oilfield Services.
In the drilling of an oil or gas well, cracks may exist in the wall of the wellbore. Such cracks may propagate, which could cause structural problems in the wall of the wellbore and/or allow drilling fluids to escape therethrough into the formation. Furthermore, if substantial amounts of drilling fluids are lost, the pressure in the drilling fluid in the wellbore may drop, which could cause collapse of the wellbore. Accordingly, a type of Lost Circulation Material known as Wellbore Strengthening Materials may be added to circulated drilling fluid. The Wellbore Strengthening Materials comprise sized particles. When the drilling fluid is circulated around the wellbore wall with cracks therein, the sized particles wedge themselves in the cracks, which reduces the likelihood of the cracks propagating. It is beneficial to recover these sized particles and reuse them in circulated drilling mud. Shale shakers have thus been modified to size solids in solids laden drilling fluid. Such a shale shaker is disclosed in U.S. Ser. No. 12/490,492. A range of sizes of solids can be extracted using such a shale shaker and re-circulated as Wellbore Strengthening Material in fresh drilling fluid.
In many conventional separators and shakers tilting or raising mechanisms are provided to adjust the angle of screen(s) with respect to the horizontal. U.S. Pat. No. 4,082,657 describes a separator apparatus which has individual height adjustable mounting structures for each screen unit which permit adjustment of the screen unit angle with respect to the horizontal. U.S. Pat. No. 6,575,304 describes a hydraulic ram apparatus beneath a screen body which is used to adjust the angle of incline of the screen body.
In many conventional systems, a determination of the level of material or fluid on a screen or screen assembly of a separator or shaker is done visually and then adjustments of screen incline angle or of screen support incline angle are done manually. A need remains for improved shaker designs, and improved control systems for shakers.
Known shale shakers generally run at a constant speed and thus as drilling mud is introduced on to the screens in the basket the acceleration value drops off.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.